This invention relates to the field of aircraft or projectile attitude determination and estimation using a GPS signal; the system using a first and second linear accelerometer for an estimation of the projectile""s roll angle, and information from an additional third accelerometer for a pitch angle estimation; the system further including means for detecting and avoiding the effects of GPS signal jamming.
The general problem to be solved is how to guide a gun-fired projectile onto a target with a known geographic location at lowest cost with reasonable reliability. This application addresses a portion of that problem by providing an estimate of the trajectory of the projectile from launch, the initial conditions of the projectile immediately after launch and the distance, direction and altitude to the idealized trajectory calculated for the projectile at the time of launch or firing and by reliably detecting and avoiding the effects of GPS signal jamming.
A first approach to the problem to be solved used only a GPS receiver and a turns counter. Although such a system represents a relatively inexpensive solution, it may be vulnerable to GPS jamming. Accordingly, additional measures may need to be implemented to ensure reliable reception and processing of an uncorrupted GPS signal, e.g., protection of the GPS signal.
A second approach used a GPS receiver, a turns counter, and a triax of gyros, which adds to the cost of the approach. Similarly to the first approach, this second approach requires protection of the GPS signal almost all the way into the target. As in the first approach, such protection is required because, in the event the GPS signal is lost, no mechanism exists to account for any external forces that may act on the projectile throughout its flight. Accordingly, a need exists for a system and process for guiding a projectile to a target while minimizing or eliminating the vulnerability of the system and process to interference with, e.g., jamming of, the GPS signal.
Prior attempts to meet this need have focused on either preventing interference with the GPS signal or enabling operation with limited GPS data. Attempts to enable operation with limited GPS data have typically involved increasing the performance and/or functionality of the inertial instruments on board the projectile. For example, to address the inability of the above-mentioned second approach to account for the projectile""s reaction to external forces if/when the GPS signal may be lost, additional accelerometers may be incorporated into the system to enable compensation for the projectile""s reaction to external forces, i.e., the sensor package may comprise a complete IMU. Gyroscopic instruments for aircraft use arc well known and available in a number of technologies such as iron rotor and tuned rotor gyros, ring laser gyros, multi-oscillator gyros, zero lock gyros (ZLG), fiber optic gyros, resonator gyros such as HRGs or hemispherical or tubular ceramic resonant gyros and the like.
Unfortunately, however, incorporation of additional gyroscopic instruments and/or accelerometers are considered much too costly because of the gyros already in the package. Moreover, use of such instruments imposes additional constraints on the operational envelopes of the projectiles. For example, gyroscopic instruments are typically subject to failure modes and uncertainties relating to launch accelerations in the range of 15,000-30,000 Gs. Further, use of these technologies usually requires that the vehicle carry at least one gyro in a gimbaled or strap-down arrangement with the attendant disadvantages of cost, weight and power dissipation.
Accordingly, it would be advantageous to have an improved, cost-effective system and process for providing projectile guidance in the presence of GPS signal jamming and/or with limited GPS data.
In accordance with a first exemplary embodiment of the invention system and process a projectile without gyros is guided toward a target using a GPS signal and a triax of accelerometers. A computer-implemented process uses GPS position and GPS delta velocity data along with data from-the accelerometer triax to determine the projectile""s estimated attitude in pitch, roll and yaw. With the projectile""s position known from data provided by the GPS signal received by a GPS receiver, the computer-implemented process determines the projectile""s attitude in navigational coordinates and creates a time indexed record of the projectile""s trajectory after the on-board GPS receiver locks on to the required number of satellites. The data in the time indexed record of the trajectory is filtered and smoothed. As the projectile rolls, the accelerometers are used to measure the forces acting on the projectile and the projectile""s rotation rates. In some alternative embodiments of the invention, the GPS is used to provide data for the calculation of initial aiming and velocity errors and for the calibration of the accelerometers when positioned in the gun barrel prior to launch.
It should be noted that the cost associated with this approach depends largely upon the accuracy of the accelerometers that must be used, which depends upon the required delivery accuracy of the projectile. Where cost must be reduced, lower accuracy accelerometers can be used with a greater reliance on GPS signal data after launch. Therefore, to reduce cost while retaining reasonable levels of projectile delivery accuracy, greater reliance is placed on GPS signal data. Thus, an aspect of the present invention provides for improved reliability of GPS signals.
In an exemplary embodiment, GPS signal data may be acquired from GPS signals transmitted from one or more satellites and received by one or more antennas on the projectile. Where the potential exists to encounter jamming of one or more of the GPS signals, one or more corresponding GPS signal jamming detector may be included to monitor each GPS signal and to detect whether each GPS signal has been subjected to jamming. If such jamming is detected, an anti-jammer will implement GPS signal protection measures.
Exemplary GPS signal protection measures may include causing the projectile to engage, or remain engaged, in a periodic motion, e.g., rolling, and using the periodic motion of the projectile to selectively sample GPS signals in such a manner as to detect and omit jammed GPS signals and to enable reliable processing of unjammed signals, e.g., by selectively and periodically avoid GPS signals that exhibit jamming. It should be noted that as a projectile proceeds through each cycle of its periodic motion, i.e., each rolling revolution, each of the one or more antennas will periodically be positioned to receive a GPS signal from each of the one or more satellites. Thus, so long as at least one GPS signal from a satellite remains capable of being received by an antenna in an unjammed condition, each of the one or more antennas will periodically be free from jamming, e.g., during at least a portion of the roll attitude.
Alternatively, where the projectile motion permits one or more antenna to remain oriented so as to continuously receive an unjammed GPS signal, such as where the projectile is not spinning, exemplary GPS signal protection measures may include detecting whether a GPS signal received by one or more of the antennas is free from jamming and switching a GPS receiver to use only the unjammed signals. It should be noted that in such an exemplary embodiment, the projectile need not be spinning or rolling. Accordingly, this protection measure may be implemented with projectiles that are not roll-stabilized. It should also be noted that, in accordance with this embodiment, the antenna(s) may be decoupled from the main body of the projectile via a mechanism such as a slip ring. Accordingly, the projectile may be spinning or roll stabilized while the antennas are maintained in a fixed orientation with respect to one or more satellites. Similarly, the projectile may be stationary while the antennas undergo periodic motion, e.g., spinning, with respect to one or more satellites.
Other exemplary protection measures may include conversion of the GPS signal from an RF signal to a digital signal as well as implementation of an automatic gain control. In such embodiments, each GPS signal may be converted by a high speed analog to digital converter immediately, or very soon, after it is received. In addition, the converted digital GPS signal may be passed through a digitally implemented automatic gain control circuit. Both the analog to digital converter and the automatic gain control circuit may be implemented in the GPS radio receiver.